U.S. patent application number 10/892237 was filed with the patent office on 2005-03-03 for stainless steel and stainless steel pipe having resistance to carburization and coking.
Invention is credited to Nishiyama, Yoshitaka, Yamadera, Yoshimi.
Application Number | 20050045251 10/892237 |
Document ID | / |
Family ID | 33475570 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050045251 |
Kind Code |
A1 |
Nishiyama, Yoshitaka ; et
al. |
March 3, 2005 |
Stainless steel and stainless steel pipe having resistance to
carburization and coking
Abstract
A stainless steel pipe includes a base metal containing 20-35
mass % of Cr, and a Cr-depleted zone is formed in the surface
region of the pipe. The Cr concentration in the Cr-depleted zone is
at least 10%, and the thickness of the Cr-depleted zone is at most
20 micrometers. A Cr-based oxide scale layer having a Cr content of
at least 50% and a thickness of 0.1-15 micrometers may be provided
on the outer side of the Cr-depleted zone. An Si-based oxide scale
layer with an Si content of at least 50% may be provided between
the Cr-based oxide scale layer and the Cr-depleted zone. The pipe
is particularly suitable for use in petroleum refineries or
petrochemical plants, such as for use as a pipe of a cracking
furnace of an ethylene plant.
Inventors: |
Nishiyama, Yoshitaka;
(Nishinomiya-shi, JP) ; Yamadera, Yoshimi;
(Kobe-shi, JP) |
Correspondence
Address: |
CLARK & BRODY
1750 K STREET NW
SUITE 600
WASHINGTON
DC
20006
US
|
Family ID: |
33475570 |
Appl. No.: |
10/892237 |
Filed: |
July 16, 2004 |
Current U.S.
Class: |
148/327 ;
148/442; 420/43; 420/584.1 |
Current CPC
Class: |
C22C 30/00 20130101;
Y10T 428/12979 20150115; Y10T 428/12847 20150115; C22C 38/02
20130101; C22C 38/40 20130101; C22C 38/04 20130101; Y10T 428/265
20150115 |
Class at
Publication: |
148/327 ;
420/043; 420/584.1; 148/442 |
International
Class: |
C22C 038/40 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 17, 2003 |
JP |
2003-276038 |
Claims
1. A stainless steel for use in a carburizing gas atmosphere
comprising a base metal including 20-55 mass % of Cr, the steel
having in its surface region a Cr-depleted zone with a Cr
concentration of at least 10% and a thickness of at most 20
micrometers.
2. A stainless steel as claimed in claim 1 further including a
Cr-based oxide scale layer with a Cr content of at least 50% on the
outer side of the Cr-depleted zone.
3. A stainless steel as claimed in claim 2 wherein the Cr-based
oxide scale layer has a thickness of 0.1-15 micrometers.
4. A stainless steel as claimed in claim 2 including an Si-based
oxide scale layer with an Si content of at least 50% between the
Cr-based oxide scale layer and the Cr-depleted zone.
5. A stainless steel as claimed in claim 3 including an Si-based
oxide scale layer with an Si content of at least 50% between the
Cr-based oxide scale layer and the Cr-depleted zone.
6. A stainless steel as claimed in claim 1 wherein the base metal
has a chemical composition comprising, in mass %, C: 0.01-0.6%, Si:
0.1-5%, Mn: 0.1-10%, P: at most 0.08%, S: at most 0.05%, Cr:
20-55%, Ni: 20-70%, N: 0.001-0.25%, O: oxygen: at most 0.02%, and a
remainder of Fe and impurities.
7. A stainless steel as claimed in claim 6 wherein the base metal
further comprises, in mass percent, at least one material selected
from the following (i)-(viii) (i) Cu: 0.01-5%, (ii) Co: 0.01-5%
(iii) At least one of Mo: 0.01-3%, W: 0.01-6%, Ta: 0.01-6%, Re:
0.01-6%, and Ir: 0.01-6% (iv) At least one of Ti: 0.01-1% and Nb:
0.01-2% (v) At least one of B: 0.001-0.1%, Zr: 0.001-0.1%, and Hf:
0.001-0.5% (vi) At least one of Mg: 0.0005-0.1%, Ca: 0.0005-0.1%,
and Al: 0.01-1% (vii) At least one of Y: 0.0005-0.15%, and Ln
series elements: 0.0005-0.15% (viii) At least one of Pd: 0.005-1%,
Ag: 0.005-1%, Pt: 0.005-1%, and Au: 0.005-1%
8. A stainless steel pipe comprising a stainless steel as claimed
in claim 1 and having surface irregularities on the inner surface
of the pipe.
9. A stainless steel pipe comprising a stainless steel as claimed
in claim 6 and having surface irregularities on the inner surface
of the pipe.
10. A stainless steel pipe comprising a stainless steel as claimed
in claim 7 and having surface irregularities on the inner surface
of the pipe.
11. A stainless steel as claimed in claim 2 wherein the base metal
has a chemical composition comprising, in mass %, C: 0.01-0.6%, Si:
0.1-5%, Mn: 0.1-10%, P: at most 0.08%, S: at most 0.05%, Cr:
20-55%, Ni: 20-70%, N: 0.001-0.25%, O: oxygen: at most 0.02%, and a
remainder of Fe and impurities.
12. A stainless steel as claimed in claim 3 wherein the base metal
has a chemical composition comprising, in mass %, C: 0.01-0.6%, Si:
0.1-5%, Mn: 0.1-10%, P: at most 0.08%, S: at most 0.05%, Cr:
20-55%, Ni: 20-70%, N: 0.001-0.25%, O: oxygen: at most 0.02%, and a
remainder of Fe and impurities.
13. A stainless steel as claimed in claim 4 wherein the base metal
has a chemical composition comprising, in mass %, C: 0.01-0.6%, Si:
0.1-5%, Mn: 0.1-10%, P: at most 0.08%, S: at most 0.05%, Cr:
20-55%, Ni: 20-70%, N: 0.001-0.25%, O: oxygen: at most 0.02%, and a
remainder of Fe and impurities.
14. A stainless steel as claimed in claim 5 wherein the base metal
has a chemical composition comprising, in mass %, C: 0.01-0.6%, Si:
0.1-5%, Mn: 0.1-10%, P: at most 0.08%, S: at most 0.05%, Cr:
20-55%, Ni: 20-70%, N: 0.001-0.25%, O: oxygen: at most 0.02%, and a
remainder of Fe and impurities.
15. A stainless steel pipe comprising a stainless steel as claimed
in claim 2 and having surface irregularities on the inner surface
of the pipe.
16. A stainless steel pipe comprising a stainless steel as claimed
in claim 3 and having surface irregularities on the inner surface
of the pipe.
17. A stainless steel pipe comprising a stainless steel as claimed
in claim 4 and having surface irregularities on the inner surface
of the pipe.
18. A stainless steel pipe comprising a stainless steel as claimed
in claim 5 and having surface irregularities on the inner surface
of the pipe.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a stainless steel having excellent
high temperature strength and corrosion resistance and having a
scale layer with an excellent ability to shield the steel against
carburizing gas. The steel is highly suitable for use in
manufacturing a steel pipe or tube (hereafter referred to as "pipe"
collectively) capable of being used in a carburizing gas atmosphere
containing hydrocarbon gas or CO gas, such as a steel pipe for a
cracking furnace, a reforming furnace, a heating furnace, or a heat
exchanger employed in a petroleum refinery or a petrochemical
plant. The present invention also relates to a stainless steel pipe
made from this material.
[0002] The present invention also relates to a method of
manufacturing a stainless steel having excellent resistance to
carburization and coking when used in a carburizing gas
atmosphere.
[0003] In recent years, due to an increasing demand for synthetic
resins, there has been a trend towards the use of higher operating
temperatures in cracking furnaces in ethylene manufacturing plants,
for example, so as to obtain a higher yield of ethylene. As a
result, pipes for use in cracking furnaces are being subjected to
higher operating temperatures. The inner surface of pipes used in
cracking furnaces are exposed to a carburizing atmosphere at high
temperatures, so the pipes need to be made of a heat resistant
material having excellent high temperature strength and resistance
to carburization.
[0004] During operation of a cracking furnace, carbon is deposited
on the inner surface of the pipes of the cracking furnace (a
phenomenon referred to as coking). As the amount of deposited
material increases, operational problems can occur such as an
increase in pressure losses (.DELTA.P) and a decrease in heating
efficiency. Accordingly, so-called decoking in which the deposited
carbon is oxidized and removed using air or steam is periodically
carried out. However, it is necessary to stop the operation of a
cracking furnace in order to perform decoking, so the operating
efficiency of the furnace is greatly decreased by the need to carry
out decoking. The problem of coking becomes worse as the diameter
of the pipes of a cracking furnace decreases. This is a major
drawback, because smaller diameter pipes are advantageous from the
standpoint of increasing product yield.
[0005] In the past, there have been various proposals of materials
for suppressing coking. For example, Japanese Published Unexamined
Patent Application Hei 2-8336 proposes a steel pipe which includes
at least 28% of Cr so as to form a strong and stable
Cr.sub.2O.sub.3 film on the surface of the pipe to prevent Fe and
Ni, which act as catalysts to promote carbon deposition, from
floating to the surface of the pipe and to thereby suppress
coking.
[0006] As disclosed in Japanese Published Unexamined Patent
Application Sho 57-23050, for example, it is known that increasing
the Si content of an alloy so as to form an SiO.sub.2 film on the
surface of the alloy is effective at increasing resistance to
carburization.
[0007] However, in the above-described prior art in which the Cr or
Si content of a steel is increased in order to form a film of
Cr.sub.2O.sub.3 or SiO.sub.2 on the steel, depending on the
operating conditions in an actual carburizing environment, a
nonuniform scale layer is formed on the steel surface. If the scale
layer undergoes cracking or peeling, it is often not possible for
the scale layer to be adequately restored (regenerated).
[0008] As a result, the scale layer does not have a satisfactory
shielding ability with respect to carburizing gas, so the problem
of needing to interrupt equipment operation in order to perform
decoking and the problem of deterioration of materials due to
carburization remain.
[0009] In order to solve these problems of nonuniform formation of
scale and inability of a scale to be regenerated, methods have been
proposed in which oxidation pretreatment is performed on a steel.
For example, Japanese Published Unexamined Patent Applications Sho
53-66832 and Sho 53-66835 disclose a method in which pretreatment
of oxidation is carried out on a 25Cr-20Ni (HK 40) low-Si heat
resistant steel or a 25Cr-35Ni low-Si heat resistant steel at
around 1000.degree. C. in air for at least 100 hours, and Japanese
Published Unexamined Patent Application Sho 57-43989 discloses a
method in which pretreatment of oxidation in air is carried out on
an austenitic heat resistant steel containing 20-35% Cr. In
addition, Japanese Published Unexamined Patent Application Hei
11-29776 discloses a method in which resistance to carburization is
increased by heating a high Ni--Cr alloy in a vacuum and forming a
scale film.
[0010] In addition, PCT-based Japanese Published Unexamined Patent
Application 2000-509105 discloses a method of increasing resistance
to carburization by performing surface treatment to form a layer
with an increased concentration of Si or Cr.
[0011] However, in any of the above-described prior art methods, it
is necessary to carry out special heat treatment or surface
treatment, so these methods are uneconomical. In addition, these
methods do not take into consideration restoration of scale (scale
regeneration) when previously oxidized scale or a surface treatment
layer peels off, so localized damage of scale is a problem.
SUMMARY OF THE INVENTION
[0012] This invention provides a stainless steel having excellent
resistance to carburization and resistance to coking due to having
the ability to form and regenerate a scale layer which shields
against carburizing gases, such as that found in pipes or tubes of
a cracking furnace for an ethylene plant. It also provides a pipe
or tube made of such a stainless steel and a method of
manufacturing such as stainless steel and pipe.
[0013] The present inventors analyzed the surface condition of
various stainless steel pipes to investigate the cause of localized
carburization and coking, even in steel pipes having a high Cr
content. It was found that the surface region of a steel pipe has a
Cr-depleted zone having a lower Cr concentration than the base
metal of the pipe.
[0014] FIG. 1 is a schematic cross-sectional view of the surface
region of a steel material having a Cr-based oxide scale layer on
its surface, showing the Cr concentration in the steel as a
function of the depth from the surface.
[0015] From this figure, it can be seen that a Cr-depleted zone is
present beneath the Cr-based oxide scale layer. The Cr-depleted
zone extends from the inner side of the oxide scale layer to where
the Cr content returns to the Cr content of the base metal.
[0016] As a result of further investigations, it was found that the
Cr-depleted zone is formed by heat treatment carried out during the
manufacture of a pipe. The heat treatment causes the formation of
an oxide scale layer on the surface of a pipe, and the Cr-depleted
zone is formed simultaneously with and immediately beneath the
oxide scale layer.
[0017] FIG. 2 is a schematic cross-sectional view of the surface
region of the steel material of FIG. 1 showing the Cr concentration
in the surface layer when the oxide scale layer has been
removed.
[0018] From in the past, it has been known that if an oxide scale
layer is formed on the surface of steel by heating, a Cr-depleted
zone is formed immediately beneath it. However, up to now it has
been thought that if the oxide scale layer is removed by shot
blasting or pickling treatment after heat treatment, the
Cr-depleted zone will also be removed. However, the present
inventors found that even after shot blasting or pickling
treatment, there are cases in which a Cr-depleted zone remains in
the surface region of a steel member.
[0019] FIG. 3 is a schematic cross-sectional view showing the Cr
concentration in the surface region of a steel material having an
Si-based oxide scale layer on the inner side of the Cr-based oxide
scale layer of FIG. 1. It was found that in this case as well in
which an Si-based oxide scale layer is formed, due to the formation
of a Cr-based oxide scale layer as an outer layer, a Cr-depleted
zone having a reduced concentration of Cr is present.
[0020] The present inventors carried out corrosion tests in a
carburizing environment using various steel pipes having such a
Cr-depleted zone. They found that in some locations a Cr-based
oxide scale layer cannot be formed, but that an oxide scale layer
containing Fe, Mn, Cr, and the like is formed, and that resistance
to carburization and resistance to coking are decreased. In the
past, the reason why carburization and coking locally occurred
during the initial period of plant operation was unclear, but the
present inventors found that the presence of a Cr-depleted zone in
the surface of a steel pipe is a primary cause.
[0021] Even with a steel pipe on which a Cr-based oxide scale layer
is formed previous to the use thereof, there are cases in which
localized carburization and coking occur. As a result of detailed
observation and analysis, it was found that carburization and
coking occur in locations where the previously formed oxide scale
layer peels off. Namely, if the Cr-based oxide scale layer peels
off, the surface of the steel on which a Cr-depleted zone is
exposed, so if a new Cr-based oxide scale layer cannot be formed,
corrosion in the form of carburization and coking occurs.
[0022] If a Cr-depleted zone is present on the surface of steel, a
Cr-based oxide scale layer is nonuniformly formed during the
initial period of plant operation. Even if a Cr-based oxide scale
layer is previously formed on the pipe during manufacturing, when
the oxide scale layer is damaged, the Cr-depleted zone is exposed
to the environment to impede regeneration of the Cr-based oxide
scale layer. In this manner, the presence of such a Cr-depleted
zone causes corrosion in the form of localized carburization and
coking.
[0023] Thus, the present inventors found that in order to achieve a
significant increase in resistance to carburization and coking, it
is important to control the characteristics of the Cr-depleted
zone.
[0024] In order to analyze the relationship between the Cr
concentration of a Cr-depleted zone in the surface region of a
steel pipe and the occurrence of carburization, test pieces (20 mm
wide by 30 mm long) were cut from steel members having Cr-depleted
zones with different Cr concentrations. The test pieces were held
for 300 hours at 1000.degree. C. in a gas atmosphere containing, in
volume percent, 15% CH.sub.4-3% CO.sub.2-82% H.sub.2 to simulate a
carburizing gas atmosphere. It was found that if the Cr
concentration in the Cr-depleted zone is less than 10%, there is an
increase in the amount of penetration of C.
[0025] In the present invention, the Cr concentration in the
Cr-depleted zone means the average Cr concentration in the
Cr-depleted zone. More specifically, the Cr concentration in the
Cr-depleted zone is the one measured with EPMA(Electron Probe Micro
Analysis).
[0026] FIG. 4 is a graph showing the relationship between the Cr
concentration in a Cr-depleted zone and the amount of penetration
of C. Here, test pieces with a Cr-depleted zone having a depth,
i.e., a thickness of 5-15 micrometers from the surface of the test
pieces were used. It can be seen that when the Cr concentration of
the Cr-depleted zone is larger than a prescribed value a
particularly marked effect on preventing carburization can be
achieved.
[0027] Based on microscopic observation of a cross section of a
test piece after the test, it was found that when the Cr
concentration of the Cr-depleted zone is less than 10%, a Cr-based
oxide scale layer cannot be formed. In order to form a Cr-based
oxide scale layer, it is necessary to supply Cr from the base metal
by diffusion, but if a Cr-depleted zone is present, the supply of
Cr becomes inadequate. As a result, instead of a Cr-based oxide
scale layer, an oxide scale layer containing Fe, Mn, Ni, Cr, or the
like is formed, but an oxide scale layer containing Fe, Mn, Ni, Cr,
or the like has a low denseness, so its ability to shield against
carburizing gas is poor. In addition, if the Fe in the oxide scale
layer is reduced and becomes metallic Fe, due to its catalyzing
effect, coking is enormously accelerated.
[0028] In order to determine the influence of the thickness of the
Cr-depleted zone, a carburizing test was carried out (the test
conditions were the same as in the case of FIG. 4). It was
ascertained that if the thickness of the Cr-depleted zone exceeds a
prescribed value, there is a tendency for the amount of C which
penetrates to increase.
[0029] FIG. 5 is a graph showing the relationship between the
thickness, i.e., depth (micrometers) of a Cr-depleted zone and the
amount of penetrated C. It uses test pieces in which the Cr
concentration of the Cr-depleted zone is 15-25 mass percent.
[0030] From this figure, it can be seen that if the thickness of
the Cr-depleted zone exceeds 20 micrometers, the amount of
penetrated C abruptly increases.
[0031] The reason for this abrupt increase is thought to be that if
the thickness exceeds a certain level, the amount of Cr supplied
from the base metal is not sufficient to form a Cr-based oxide
scale layer on the surface of the steel having the ability to
shield against carburizing gas during plant operation.
[0032] Next, analysis of a Cr-based oxide scale layer was carried
out using a steel pipe on the surface of which a Cr-based oxide
scale layer (A) was previously formed. It was found by experiment
that if the Cr content in the oxide scale layer is at least 50% and
preferably at least 80%, carburization is suppressed.
[0033] FIG. 6 is a graph showing the relationship between the Cr
concentration in the oxide scale layer and the amount of C which
penetrates.
[0034] This figure was obtained using test pieces in which the Cr
concentration of the Cr-depleted zone was 15-25 mass percent, the
thickness of the Cr-depleted zone was approximately 10 micrometers,
and the thickness of the oxide scale layer on the surface of the
test pieces was 2-7 micrometers.
[0035] As shown in FIG. 6, if the Cr concentration in the scale
layer is greater than or equal to 50%, there is an abrupt decrease
in the amount of penetrated C. In addition, from microscopic
observation of cross sections of test pieces after the test, it was
observed that the oxide scale layer is dense, so it is thought that
it has excellent ability to shield against carburizing gas. In
addition, it became clear that it is difficult for cracking and
peeling of the oxide scale layer to occur.
[0036] It was found that the thickness of a Cr-based oxide scale
layer has an influence on the shielding abilities and on damage
such as cracking and peeling. Namely, if the thickness of the
Cr-based oxide scale layer is small, the shielding properties are
not sufficient, while if the scale thickness is too great, it
becomes easy for damage such as cracking and peeling to occur. This
is thought to be because as the thickness of the scale layer
increases, growth stress in the oxide scale layer increases, and
cracking and peeling occur in order to alleviate this stress.
[0037] The present inventors found that by forming an Si-based
oxide scale layer (B) in the interface between the Cr-based oxide
scale layer (A) and the stainless steel base metal, not only is the
uniform formation of the oxide scale layer (A) in the initial
period of operation promoted, but when damage such as cracking and
peeling of the oxide scale layer (A) occurs, the Si-based oxide
scale layer (B) promotes regeneration of damaged portions of oxide
scale layer (A). However, even when such an Si-based oxide scale
layer (B) is present, unless the Cr concentration and the thickness
of the Cr-depleted zone are appropriate, localized corrosion
occurs.
[0038] According to one form of the present invention, a stainless
steel for use in a carburizing atmosphere has a base metal
containing 20-55 mass % of Cr. The steel includes a Cr-depleted
zone in its surface region. The Cr-depleted zone has a Cr
concentration of at least 10% and a thickness of at most 20
micrometers.
[0039] The stainless steel may further include a Cr-based oxide
scale layer with a Cr content of at least 50% formed on the outer
side of the Cr-depleted zone.
[0040] The oxide scale layer will typically have a thickness of
0.1-15 micrometers.
[0041] The stainless steel may further include an Si-based oxide
scale layer with an Si content of at least 50% between the Cr-based
oxide scale layer and the Cr-depleted zone.
[0042] The base metal preferably has a chemical composition
comprising, in mass percent,
[0043] C: 0.01-0.6%, Si: 0.1-5%, Mn: 0.1-10%, P: at most 0.08%, S:
at most 0.05%, Cr: 20-55%, Ni: 20-70%, N: 0.001-0.25%, O: oxygen:
at most 0.02%, and a remainder of Fe and impurities.
[0044] The base metal may further comprise, in mass percent, at
least one material selected from the following (i)-(viii):
[0045] (i) Cu: 0.01-5%,
[0046] (ii) Co: 0.01-5%
[0047] (iii) At least one of Mo: 0.01-3%, W: 0.01-6%, Ta: 0.01-6%,
Re: 0.01-6%, and Ir: 0.01-6%
[0048] (iv) At least one of Ti: 0.01-1% and Nb: 0.01-2%
[0049] (v) At least one of B: 0.001-0.1%, Zr: 0.001-0.1%, and Hf:
0.001-0.5%
[0050] (vi) At least one of Mg: 0.0005-0.1%, Ca: 0.0005-0.1%, and
Al: 0.01-1%
[0051] (vii) At least one of Y: 0.0005-0.15%, and Ln series
elements: 0.0005-0.15%
[0052] (viii) At least one of Pd: 0.005-1%, Ag: 0.005-1%, Pt:
0.005-1%, and Au: 0.005-1%
[0053] According to another form of the present invention, a
stainless steel pipe comprises the above-described stainless steel
and has a plurality of fins and bosses on its inner surface.
[0054] According to yet another form of the present invention, a
method of improving resistance to carburization and coking of a
stainless steel pipe for use in a carburizing gas atmosphere
employs a pipe with a base metal including 20-55 mass % of Cr. The
method includes providing a Cr-depleted zone in the surface region
of the steel pipe. The Cr concentration of the Cr-depleted zone is
at least 10%, and the thickness of the Cr-depleted zone at most 20
micrometers.
[0055] A Cr-based oxide scale layer having a Cr content of at least
50% may be provided on the outer side of the Cr-depleted zone, with
the thickness of the oxide scale layer preferably being 0.1-15
micrometers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] FIG. 1 is a schematic cross-sectional view of the surface
region of a steel material having a Cr-based oxide scale layer on
the surface, showing the Cr concentration of the steel as a
function of depth from the surface of the steel.
[0057] FIG. 2 is a schematic cross-sectional view of the surface
region of the steel material of FIG. 1 showing the Cr concentration
in the surface region when the oxide scale layer of FIG. 1 has been
removed.
[0058] FIG. 3 is a schematic cross-sectional view of the surface
region of a steel material having an Si-based oxide scale layer on
the inner side of the Cr-based oxide scale layer of FIG. 1, showing
the Cr concentration in the surface region.
[0059] FIG. 4 is a graph of the relationship between the Cr
concentration of a Cr-depleted zone and the increase of C
content.
[0060] FIG. 5 is a graph of the relationship between the depth of a
Cr-depleted zone and the increase of C content.
[0061] FIG. 6 is a graph of the relationship between the Cr
concentration of an oxide scale layer and the increase of C
content.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0062] Next, the reasons for the ranges of various parameters of
the present invention will be explained. In the following
explanation, unless otherwise indicated, percent when used to
describe chemical composition refers to mass percent.
[0063] A stainless steel according to the present invention
comprises a base metal including 20-55% Cr and preferably 20-35%
Cr. A stainless steel comprising 20-35% Cr is suitable for use in
manufacturing pipes for ethylene manufacture (ethylene cracking
tubes).
[0064] (i) Cr-depleted Zone
[0065] Cr concentration of the Cr-depleted zone: The Cr-depleted
zone is formed immediately below the oxide scale layer which is
formed during homogenizing heat treatment of a stainless steel
according to the present invention. The Cr concentration of this
Cr-depleted zone is lower than the Cr concentration of the base
metal, but if it is less than 10%, a Cr-based oxide scale layer
having the ability to shield against carburizing gas during plant
operation cannot be formed on the surface of the steel. When a
Cr-based oxide scale layer already exists on the steel surface
prior to the use thereof, if the Cr concentration of the
Cr-depleted zone immediately beneath it is less than 10%, the
Cr-based oxide scale layer cannot be regenerated if it undergoes
damage such as cracking or peeling. Preferably the Cr concentration
of the Cr-depleted zone is at least 12%.
[0066] Thickness of the Cr-depleted zone: The Cr-depleted zone is
formed immediately below the oxide scale layer which is formed
during homogenizing heat treatment. If the thickness of the
Cr-depleted zone exceeds 20 micrometers, it is difficult to form a
Cr-based oxide scale layer on its surface which has the ability to
shield against carburizing gas during plant operation. Therefore,
the thickness of the Cr-depleted zone is made at most 20
micrometers. Preferably the thickness is at most 15
micrometers.
[0067] The thickness of the Cr-depleted zone can be easily adjusted
by heat treatment in a controlled atmosphere, for example.
[0068] The Cr concentration and thickness of the Cr-depleted zone
can be measured with EPMA. A specimen for EPMA can be prepared by
cutting a specimen with a microscopic cross section, polishing it
with emery paper, buffing with alumina powder, and performing
degreasing. In EPMA, vapor deposition of C is typically performed
on the surface of a specimen, and the Cr concentration in the depth
direction is measured while moving the probe at the rate of 2-400
micrometers a minute. In the measurement with EPMA, acceleration
voltage is at 10-25 KeV (preferably 15-20 KeV) and electric current
at 5-30 nA (preferably 5-20 nA).
[0069] (ii) Oxide Scale Layer
[0070] Composition of Oxide Scale Layer (A):
[0071] The Cr-based oxide scale layer is extremely important for
providing resistance to carburization and coking. A Cr-based oxide
scale layer with a Cr content of at least 50% has a high denseness
and good ability to shield against penetration of carbon into
steel. In addition, a Cr-based oxide scale layer has a small
catalyzing effect with respect to coking, so it suppresses coking
of the steel surface. As a result, it maintains the thermal
conductivity of the pipe with respect to fluids inside it for long
periods, and the yield of reaction products such as olefins is
stabilized.
[0072] If the Cr content of the oxide scale layer is at least 80%,
the scale layer becomes denser, and a shielding layer which has
good resistance to penetration of carbon into steel is obtained. As
a result, resistance to carburization is dramatically increased. A
more preferred Cr content is at least 82%, and a still more
preferred Cr content is at least 85%.
[0073] Thickness of Oxide Scale Layer (A):
[0074] The thickness of the Cr-based oxide scale layer is an
important factor affecting penetration of carbon into steel. The
effect of the Cr-based oxide scale layer as a shielding layer is
small if its thickness is less than 0.1 micrometer. On the other
hand, if its thickness exceeds 15 micrometers, growth stress and
thermal stress at the time of cooling accumulate, and cracking and
peeling of the oxide scale layer occur, so it becomes easy for
carbon to penetrate the steel. Therefore, the thickness of oxide
scale layer (A) is preferably 0.1-15 micrometers. In order to
obtain shielding properties with greater certainty, the thickness
of oxide scale layer (A) is preferably 0.5-15 micrometers and most
preferably 0.5-10 micrometers.
[0075] The formation of such an oxide scale layer can easily be
achieved by, for example, heat treatment in an atmosphere of a
controlled combustion gas.
[0076] Oxide Scale Layer (B):
[0077] An Si-based oxide scale layer (B) having an Si content of at
least 50% may be formed between the Cr-depleted zone and the
Cr-based oxide scale layer (A). Oxide scale layer (B) promotes the
uniform formation of oxide scale layer (A), and in addition, when
there is damage of oxide scale layer (A) such as cracking and
peeling, oxide scale layer (B) promotes regeneration of the damaged
portion.
[0078] Oxide scale layer (B) can be easily formed by increasing the
Si content of the base metal steel.
[0079] The chemical composition of oxide scale layer (A) and oxide
scale layer (B) can be measured by EDX (Energy Dispersive X-ray
spectrometry). A test specimen can be prepared by the
above-described procedure, for example. In EDX, vapor deposition of
C is typically performed on the surface of the test specimen, and
then quantitative elemental analysis is performed. The thickness of
oxide scale layer (B) can be measured by observing a microscopic
sample of a cross section with on optical microscope.
[0080] The inner surface of a steel pipe according to the present
invention may have surface irregularities, such as bosses or fins
for increasing the surface area. Here, surface irregularities refer
to departures of the shape of the inner surface of the pipe from a
perfectly cylindrical shape which are significantly larger than the
surface roughness of the inner surface of the pipe. Bosses, fins,
or other surface irregularities may be integrally formed with the
pipe body, or they may be attached to the inner surface by welding
or other method. The surface irregularities may be randomly
arranged on the inner surface, or they may be arranged in a regular
pattern. Normally, it is thought that the provision of surface
irregularities on a surface makes it easier for an oxide scale
layer to be damaged by carburizing gas and undergo peeling.
However, according to the present invention, because the resistance
to carburization of the inner surface of the steel pipe is high and
the oxide scale layer has a good ability of self-healing, the
provision of surface irregularities does not in any way reduce the
resistance to carburization and coking of the steel pipe.
[0081] A stainless steel having the following composition is
preferred as the base metal of the steel according to the present
invention. The reasons for the limits on the chemical composition
of the base metal of the stainless steel are as follows.
[0082] C: 0.01-0.6%
[0083] At least 0.01% of C is included in the steel according to
the present invention in order to guarantee high temperature
strength. If the C content exceeds 0.6%, the toughness of the
stainless steel becomes extremely poor, so the upper limit is made
0.6%. Preferably the C content is 0.02%-0.45% and more preferably
0.02-0.3%.
[0084] Si: 0.1-5%
[0085] Si has a strong affinity for oxygen, so it promotes uniform
formation of a Cr-based oxide scale layer (A). This effect is
exhibited if the Si content is at least 0.1%. However, if the Si
content exceeds 5%, weldability worsens and the microstructural
stability worsens, so the upper limit of the Si content is made 5%.
A preferred range for the Si content is 0.1-3%, and a more
preferred range is 0.3-2%.
[0086] Mn: 0.1-10%
[0087] Mn is added order for the purposes of deoxidizing and
improving workability. For these purposes, at least 0.1% is added.
Mn is an austenite forming element, so it is possible to replace a
portion of Ni with Mn, but addition of too much Mn impedes the
formation of a Cr-based oxide scale layer, so the upper limit on
the Mn content is made 10%. A preferred range for Mn is 0.1-5% and
a more preferred range is 0.1-2%.
[0088] P: At most 0.08%, S: at most 0.05%
[0089] P and S segregate at grain boundaries and worsen hot
workability. Therefore, they are preferably reduced as much as
possible, but an excessive decrease leads to an increase in costs,
so the P content is made at most 0.08%, and the S content is made
at most 0.05%. The P content is preferably at most 0.05% and more
preferably at most 0.04%, and the S content is preferably at most
0.03% and more preferably at most 0.015%.
[0090] Cr: 20-55%
[0091] Cr is an important element in the present invention. It is
necessary for the Cr content to be at least 20% in order to stably
form a Cr-based oxide scale layer. However, addition of too much Cr
decreases pipe manufacturability and decrease the microstructural
stability during use of a pipe at high temperatures, so the upper
limit on the Cr content is made 55%. In order to prevent a
deterioration in workability and stability of metallurgical
structure, the upper limit on the Cr content is preferably 35%. A
more preferred range is 22-33%.
[0092] Ni: 20-70%
[0093] The addition of Ni is necessary in order to obtain a
stabilized austenite structure containing Cr. For this purpose, the
Ni content needs to be 20-70%. Another benefit of the addition of
Ni is that it reduces the speed of penetration of C into the steel.
However, addition of more Ni than is necessary leads to cost
increases and difficulty in manufacturing. A preferred range for
the Ni content is 20-60%, and a more preferred range is 23-50%.
[0094] N: 0.001-0.25%
[0095] N is effective at improving high temperature strength. It is
necessary for the N content to be at least 0.001% in order to
obtain this effect. Addition of too much N greatly impairs
workability, so the upper limit on the N content is made 0.25%.
Preferably the N content is 0.001%-0.2%.
[0096] Oxygen (O): at most 0.02%
[0097] Oxygen (O) is present in a steel according to the present
invention as an impurity. If the oxygen content exceeds 0.02%, a
large amount of oxide inclusions are present in the steel, so
workability is decreased, and in addition, surface defects may
occur in the steel pipe, so the upper limit on the oxygen content
is made 0.02%.
[0098] The following elements may also be added to a steel
according to the present invention.
[0099] Cu: 0.01-5%
[0100] Cu stabilizes an austenite phase, and it is effective for
increasing high temperature strength, so at least 0.01% may be
added. On the other hand, if it is added in excess of 5%, hot
workability is markedly decreased, so the Cu content is made
0.01-5%. A preferred range for the Cu content is 0.01-3%.
[0101] Co: 0.01-5%
[0102] Co stabilizes an austenite phase, so it can replace a
portion of Ni. If Co is added in excess of 5%, hot workability is
markedly decreased, so it is made 0.01-5%. A preferred range for
the Co content is 0.01-3%.
[0103] At least one of Mo: 0.01-3%, W: 0.01-6%, Ta: 0.01-6%, Re:
0.01-6%, and Ir: 0.01-6%.
[0104] Each of Mo, W, Ta, Re, and Ir is a solid solution
strengthening element and is effective for increasing high
temperature strength. In order to obtain these effects, it is
necessary to add at least 0.01% each of any of these which is
added. However, excessive addition deteriorates workability and
impairs the stability of the metallurgical structure, so the upper
limit for the content of Mo is at most 3%, and the upper limit for
the content of W, Ta, Re, and Ir is at most 6%. The preferred range
for any of Mo, W, Ta, Re, and Ir which is added is 0.01-2.5%, and a
more preferred range is 0.01-2%.
[0105] At least one of Ti: 0.01-1% and Nb: 0.01-2%
[0106] Ti and Nb have a significant effect on improving high
temperature strength, ductility, and toughness even when added in
minute amounts. However, neither of these elements can provide
these effects if the content of either of these which is added is
less than 0.01%, while workability and weldability decrease if the
Ti content exceeds 1% or the Nb content exceeds 2%.
[0107] At least one of B: 0.001-0.1%, Zr: 0.001-0.1%, and Hf:
0.001-0.5%
[0108] Each of B, Zr, and Hf is effective at strengthening of grain
boundaries and improving hot workability and high temperature
strength. However, these effects are not obtained with less than
0.001% each of any of these which is added, while excessive
addition decreases weldability, so the range for each of these
elements which is added is 0.001-0.1%, 0.001-0.1%, and 0.001-0.5%,
respectively.
[0109] At least one of Mg: 0.0005-0.1%, Ca: 0.0005-0.1%, and Al:
0.01-1%
[0110] Each of Mg, Ca, and Al is effective at improving hot
workability. When they are added, the lower limit on the content
for providing these effects is at least 0.0005% for Mg and Ca and
at least 0.01% for Al. However, addition of too much decreases
weldability, so the upper limits are 0.1% for Mg and Ca and 1% for
Al. Preferred ranges are 0.0008-0.05% for Mg and Ca and 0.01-0.6%
for Al.
[0111] At least one of Y and Ln series elements: 0.005-0.15%
[0112] Y and Ln series elements are effective at increasing
oxidation resistance, so a stainless steel according to the present
invention may include Y and/or one or more Ln series elements. The
effects thereof are not obtained with less than 0.005% of any of
these which is added, while excessive addition worsens workability,
so the upper limit for each is made 0.15%. Of Ln series elements,
it is particularly preferred to use one or more of La, Ce, and Nd.
The Ln series refers to the elements La (atomic number 57) through
Lu (atomic number 71) on the periodic table.
[0113] At least one of Pd: 0.005-1%, Ag: 0.005-1%, Pt: 0.005-1%,
and Au: 0.005-1%
[0114] Each of Pd, Ag, Pt, and Au can be added with the object of
increasing corrosion resistance. The effect thereof cannot be
obtained with less than 0.005% of any one which is added, whereas
addition of more than 1% decreases workability and leads to an
increase in costs, so the upper limit for each is made 1%. The
preferred range for any of Pd, Ag, Pt, and Au which is added is
0.005-0.5%.
[0115] Although both the inner and outer surfaces of a stainless
steel pipe according to the present invention may have the ability
to form and regenerate a scale layer which shields against
carburizing gas, typically only the inner surface of the pipe is
exposed to carburizing gas during use. Therefore, in most
situations, it is sufficient if just the inner surface of the pipe
has the ability to form and regenerate a scale layer which shields
against carburizing gas.
[0116] A stainless steel according to the present invention can be
formed into a pipe by conventional methods used for pipe
manufacture, including steps such as melting, casting, hot working,
cold working, and welding. It may be either a seamless pipe or a
welded pipe. It can also be formed into a pipe by methods such as
powder metallurgy methods or centrifugal casting. The manufacturing
method will typically include final heat treatment which produces a
Cr concentration of the Cr-depleted zone of at least 10%. After
final heat treatment is carried out, surface treatment such as
pickling, shot blasting, machining, grinding, and electropolishing
may be carried out on the surface of the steel pipe.
[0117] Formation of oxide scale layers (A) and (B) is carried out
at the time of the final heat treatment. The desired oxide scale
layers result from a suitable combination of the steel composition
and the heat treatment conditions, as will be readily understood by
those skilled in the art from the proceeding explanation.
EXAMPLES
[0118] The present invention will be described in greater detail by
the following examples, which are meant to be illustrative and do
not limit the scope of the present invention.
[0119] Steels having the chemical compositions shown in Table 1
were melted in a high frequency vacuum heating furnace and formed
into billets. The resulting billets were subjected to hot forging
and cold rolling to prepare steel pipes with an outer diameter of
56 mm and a wall thickness of 6 mm. Each steel pipe underwent heat
treatment under one of the four heat treatment conditions A-B
described below. After heat treatment, the steel pipes were cut
open, and some of the pipes were subjected to surface treatment in
the form of shot blasting, pickling, or machining, while the
remaining pipes were left in an as heat treated condition. For
steel numbers 1-3 and 24 in Table 1, for each of the heat treatment
conditions, heat treatment was carried out at 1200.degree. C. for
10 minutes. For steel numbers 4-23, heat treatment was carried out
using heat treatment condition A while varying the heat treatment
temperature in the range of 1000-1250.degree. C. and varying the
heat treatment time in the range of 1 minute to 1 hour.
[0120] Heat Treatment Condition A:
[0121] vacuum heat treatment (1000-1250.degree. C.) for 1 minute to
1 hour
[0122] Heat Treatment Condition B:
[0123] heat treatment in a gas containing 20 vol % H.sub.2O
(1050-1250.degree. C.) for 1 minute to 1 hour
[0124] Heat treatment condition C:
[0125] two-step heat treatment (heat treatment condition A+heat
treatment condition B)
[0126] Heat treatment condition D:
[0127] two-step heat treatment (heat treatment condition B+heat
treatment condition A)
[0128] Test pieces measuring 20 mm on a side (20 mm.times.20
mm.times.6 mm) were cut from the steel pipes which were subjected
to the surface treatment, the test pieces were worked to prepare
test pieces for observation of the cross section, and the Cr
concentration in the Cr-depleted zone and the thickness of the
Cr-depleted zone were measured with EPMA (Electron Probe
Micro-Analysis). For the "as heat treated" steel pipes which did
not undergo surface treatment, an oxide scale layer remained on the
steel surface, so the Cr content of the oxide scale layer and the
thickness of the oxide scale layer were measured by EDX and a light
microscope, respectively, and the Cr concentration and thickness of
the Cr-depleted zone were measured by the same method as for the
steel pipes which underwent surface treatment.
[0129] The results are compiled in Table 2.
[0130] Test pieces having a width of 20 mm and a length of 30 mm
were cut from steel pipes which underwent the same heat treatment
and surface treatment as the test pieces described with respect to
Table 2. These test pieces were held for 300 hours at 1000.degree.
C. in a gas atmosphere containing, in volume %, 15% CH.sub.4-3%
CO.sub.2-82% H.sub.2 and a test of coking properties was carried
out. Coking properties were evaluated based on the amount of C
which penetrated the base metal after holding in the
above-described gas atmosphere. Namely, metal cuttings were
obtained from the test pieces at a pitch of 5 mm in the depth
direction from the surface, and the amount of C (mass %) at a depth
of 0.5-1.0 mm and a depth of 1.0-1.5 mm was measured by chemical
analysis of the metal cuttings. After the amount of C in the base
metal (mass %) prior to the test was subtracted, the average value
of both amounts of C was made the amount of C (mass %) which
penetrated to a depth of 1 mm.
[0131] The results are compiled in Table 3.
[0132] As shown in Table 3, a steel pipe of steel number 24 for
which the chemical composition was outside the range of the present
invention had a large amount of penetration of C and a large amount
of surface accumulation of C for both heat treatment condition A
and B, and its resistance to carburization and resistance to coking
were both poor.
[0133] As also shown in Table 3, of the steel pipes made of steels
number 1-38 which satisfied the chemical composition set forth in
the present invention, those which satisfied the requirements for
the Cr concentration and the thickness of the Cr-depleted zone
according to the present invention had an extremely small amount of
penetrated C and surface accumulation of C, and the resistance to
carburization and resistance to coking were excellent, but for the
steel pipes of the steel numbers which did not satisfy one or both
of the conditions of the present invention for the Cr concentration
and the thickness of the Cr-depleted zone, the amount of
penetration of C and the amount of surface accumulation of C were
large, and the resistance to carburization and the resistance to
coking were inferior.
1TABLE 1 Steel Chemical composition of base metal (mass %) No. C Si
Mn P S Cr Ni N Oxygen Others 1 0.21 0.36 0.42 0.020 <0.001 25.8
24.5 0.04 0.010 0.5Ti 2 0.11 1.67 0.28 0.017 <0.001 25.3 38.3
0.02 0.010 1.2Mo 3 0.08 0.35 1.20 0.025 <0.001 20.7 30.5 0.02
0.003 0.004Ca 4 0.11 0.87 0.55 0.035 0.035 26.4 37.9 0.02 0.017
2.9Co 5 0.06 1.67 0.34 0.018 <0.001 25.3 37.6 0.21 0.004 0.034Ce
6 0.13 0.54 0.66 0.021 0.001 26.4 34.2 0.03 0.009 0.12Al 7 0.04
3.55 0.44 0.015 0.001 24.8 33.8 0.04 0.005 0.02Zr, 0.3Ti 8 0.16
1.11 0.84 0.065 <0.001 26.7 38.5 0.02 0.005 0.025Y 9 0.06 0.85
0.77 0.018 0.001 22.5 23.5 0.02 0.010 -- 10 0.08 1.45 1.35 0.025
0.002 23.8 46.5 0.03 0.010 3.5W 11 0.13 0.32 0.16 0.024 0.002 23.8
36.4 0.13 0.006 2.5Cu 12 0.11 1.85 3.20 0.022 0.001 28.9 42.5 0.05
0.015 1.3Nb 13 0.01 0.12 0.15 0.018 <0.001 31.2 60.8 0.01 0.005
0.029La 14 0.07 0.55 0.32 0.030 0.003 26.1 40.1 0.03 0.010 0.2W,
0.3Mo 15 0.04 1.59 0.28 0.027 0.001 24.2 43.1 0.06 0.010 0.008 B 16
0.32 0.16 0.88 0.042 0.027 23.1 32.1 0.01 0.007 0.06Zr 17 0.09 0.57
0.59 0.049 0.001 24.6 35.8 0.01 0.007 0.05Hf 18 0.11 1.12 0.24
0.022 0.005 22.1 32.5 0.03 0.007 0.004Mg 19 0.02 1.33 1.09 0.029
0.011 23.9 36.8 0.02 0.010 0.041Nd 20 0.10 1.13 0.89 0.030 0.021
24.0 40.8 0.01 0.015 0.2Cu, 1.2Co 21 0.09 1.25 1.20 0.009 0.003
25.2 33.5 0.03 0.010 1.4Cu, 0.13Nd 22 0.06 1.34 0.43 0.021 0.002
25.3 40.3 0.03 0.010 2.5Co, 2.8W 23 0.01 1.35 1.31 0.029 0.009 22.8
39.5 0.02 0.005 3.1Cu, 0.59Co, 0.9Mo 0.4Ti, 0.018B, 0.010Mg, 0.031Y
24 0.11 0.46 1.31 0.025 0.001 18.6 25.5 0.03 0.010 -- 25 0.07 0.51
0.39 0.015 0.001 25.0 34.5 0.04 0.010 0.5Ti, 0.5Al, 0.4Re 26 0.05
1.64 1.51 0.015 0.001 25.3 35.5 0.16 0.010 0.05Ce, 0.02Pd 27 0.45
1.82 1.10 0.021 0.002 31.5 44.2 0.02 0.015 1.13Nb, 0.1Pt 28 0.47
1.78 1.15 0.020 0.002 26.1 35.4 0.03 0.013 0.7Nb, 0.31r 29 0.09
1.81 0.51 0.015 0.001 25.3 42.1 0.01 0.007 0.2Ti, 0.4Nb, 0.2Ta
0.1Ag 30 0.25 0.48 0.28 0.021 0.001 44.8 52.1 0.01 0.011 -- 31 0.07
1.57 1.12 0.022 0.001 23.5 35.8 0.03 0.008 0.12Au 32 0.12 0.15 0.22
0.015 0.001 23.7 45.1 0.02 0.005 0.9Al, 0.03Pr 33 0.06 1.54 0.32
0.008 0.001 28.9 57.6 0.01 0.009 1.3Ta 34 0.08 1.67 0.45 0.011
0.001 24.2 38.7 0.02 0.004 1.1Re 35 0.12 1.27 0.67 0.009 0.002 23.1
36.7 0.02 0.008 0.8Ir 36 0.15 1.81 0.11 0.015 0.001 22.8 37.1 0.01
0.004 0.3Pd 37 0.11 1.38 0.71 0.019 0.002 26.4 34.9 0.02 0.007
0.2Ag 38 0.15 0.87 0.38 0.024 0.001 27.1 39.1 0.02 0.004 0.3Pt
Underlining indicates a value outside the range of the present
invention
[0134]
2 TABLE 2 Cr-depleted Oxide scale Oxide scale zone layer (A) layer
(B) heat Cr Cr Thick- Si Thick- Steel treatment Surface
concentration Depth content ness content ness No. condition
treatment (mass %) (.mu.m) (mass %) (.mu.m) (mass %) (.mu.m) 1 A
shot blasting 14.7 10 -- -- -- -- B shot blasting 9.4 12 -- -- --
-- 2 A as heat treated 16.2 10 96 4 80 0.5 B as heat treated 18.7
24 90 6 80 0.5 C as heat treated 13.1 8 74 9 85 0.8 D as heat
treated 14.5 18 82 17 75 0.5 3 A as heat treated 10.9 14 82 9 55
0.4 B as heat treated 6.8 22 80 13 75 0.7 C shot blasting 12.1 10
-- -- -- -- D shot blasting 7.8 10 -- -- -- -- 4 A shot blasting
18.3 8 -- -- -- -- 5 A pickling 17.3 5 -- -- -- -- 6 A as heat
treated 15.5 15 92 9 50 0.3 7 A pickling 21.4 4 -- -- -- -- 8 A
shot blasting 24.6 10 -- -- -- -- 9 A pickling 17.8 10 -- -- -- --
10 A machining 20.9 2 -- -- -- -- 11 A as heat treated 14.2 12 90 7
30 0.3 12 A shot blasting 26.8 3 -- -- -- -- 13 A pickling 24.5 5
-- -- -- -- 14 A shot blasting 20.5 7 -- -- -- -- 15 A as heat
treated 14.6 9 93 4 80 0.4 16 A machining 21.5 5 -- -- -- -- 17 A
pickling 21.4 4 -- -- -- -- 18 A pickling 18.6 5 -- -- -- -- 19 A
shot blasting 20.2 5 -- -- -- -- 20 A as heat treated 15.6 6 80 9
75 0.5 21 A as heat treated 13.8 8 80 10 95 0.8 22 A as heat
treated 18.1 5 90 7 90 0.7 23 A as heat treated 12.5 10 75 12 90
0.8 24 A as heat treated 6.2 14 73 12 40 0.5 B shot blasting 8.9 7
-- -- -- -- 25 A as heat treated 16.2 10 75 11 30 0.2 26 A as heat
treated 16.4 12 90 8 90 0.6 27 A as heat treated 21.5 12 88 8 90
0.7 28 A as heat treated 17.2 11 85 7 90 0.6 29 A as heat treated
15.4 14 85 9 90 0.5 30 A as heat treated 27.5 16 95 8 30 0.4 31 A
as heat treated 15.8 10 88 8 90 0.7 32 A as heat treated 18.6 10 70
6 -- -- 33 A as heat treated 22.3 10 93 6 80 0.5 34 A as heat
treated 15.1 10 80 8 80 0.6 35 A as heat treated 13.0 16 74 10 75
0.4 36 A as heat treated 11.8 17 75 10 90 0.6 37 A as heat treated
14.8 11 80 8 70 0.3 38 A as heat treated 18.9 13 93 9 50 0.7
Underlining indicates a value outside the ragge of the present
invention
[0135]
3 TABLE 3 Heat Increase in Amount of Steel treatment C content coke
deposition No. condition (mass %) (mg/cm.sup.2) 1 A 0.9 1.8 B 2.2
8.9 2 A 0.6 1.0 B 1.7 6.2 C 0.9 1.2 D 0.9 1.3 3 A 1.1 1.9 B 2.8
12.5 C 1.2 1.5 D 2.7 9.7 4 A 0.6 0.5 5 A 0.4 0.5 6 A 0.8 1.5 7 A
0.3 0.8 8 A 0.45 0.5 9 A 1.2 1.7 10 A 0.6 0.6 11 A 1.4 2.3 12 A 0.4
0.3 13 A 0.5 0.6 14 A 0.7 0.9 15 A 0.8 0.6 16 A 0.7 0.6 17 A 1.4
1.3 18 A 0.6 0.6 19 A 0.55 0.6 20 A 0.6 0.3 21 A 0.9 0.9 22 A 0.4
0.2 23 A 1.2 1.3 24 A 3.3 15.3 B 3.4 12.4 25 A 1.3 1.5 26 A 0.9 0.8
27 A 0.5 0.3 28 A 0.9 0.8 29 A 0.7 0.6 30 A 0.5 0.2 31 A 0.8 0.6 32
A 0.4 0.4 33 A 0.4 0.4 34 A 0.6 0.5 35 A 1.3 1.2 36 A 1.1 0.8 37 A
0.8 0.8 38 A 0.7 0.6 Underlining indicates a value outside the
range of the present invention
[0136] As described above, a steel according to the present
invention has the ability to form and regenerate a surface scale
layer which shields against carburizing gas, and it has excellent
resistance to carburization and coking, so pipes made from this
steel can be used in cracking furnaces, reforming furnaces, heating
furnaces, piping, and heat exchangers in petroleum refineries and
petrochemical plants. Therefore, the pipes can greatly increase the
durability and the operating efficiency of equipment.
* * * * *